It is known in the art to produce supported silver catalysts for the conversion of ethylene and oxygen to ethylene oxide. Many modifications have been proposed to improve the activity and selectivity of these catalysts. These modifications have involved improvements to the supports employed, the methods of production, the physical form of the silver on the support and the inclusion of additives to the catalyst composition. Methods are known for the preparation of supported silver catalysts useful for the vapor phase oxidation of ethylene to ethylene oxide, which involve impregnating a support such as alumina with a silver salt/amine solution. U.S. Pat. No. 3,702,359 is illustrative of such procedures.
U.S. Pat. No. 2,125,333 discloses the use of alkali metals, including sodium or potassium and their salts as additives for various silver ethylene oxide catalysts. U.S. Pat. No. 2,615,900 cites a large number of useful promoters for silver catalysts. U.S. Pat. No. 2,773,844 discloses a multi-step silver deposition process for preparing a silver based catalyst. U.S. Pat. No. 3,575,888 discloses the use of aluminum oxide supports having a pore volume between about 0.15 and 0.30 m2/gm and surface area below about 10 m2/gm. The use of small amounts of alkali metals, K, Rb and Cs, were noted as useful promoters in supported silver catalysts. See, for example, U.S. Pat. Nos. 3,962,136 and 4,010,115.
U.S. Pat. No. 4,005,049 teaches the preparation of a silver/transition metal catalyst useful in oxidation reactions. In the '049 patent, catalytically active metals such as Ag and Re are co-sputtered along with a co-sputtered support material on a particular support. The preparation of silver catalysts which also contain alkali metal promoters by analogous procedures is shown, for example, in U.S. Pat. No. 3,962,136. Similar procedures for the preparation of silver catalysts promoted by an alkali metal and rhenium and also with a co-promoter selected from sulfur, molybdenum, tungsten, chromium and mixtures are disclosed, for example, in U.S. Pat. No. 4,766,105.
Catalyst preparation by these prior art procedures has involved impregnating a support with a silver/amine solution which may contain various promoters, and thereafter heating the impregnated support in a forced air oven up to a temperature of about 275° C. in order to reduce the silver to metallic silver and to separate volatiles from the catalyst. This step of reducing the silver to metallic silver is typically referred to in the art as calcination.
U.S. Pat. No. 4,916,243 show silver catalysts for ethylene oxidation to ethylene oxide prepared by impregnating an inert support with a silver/amine and silver lactate solutions. The impregnated carriers were then heat treated on a steel belt transported through a 2″×0.2″ square heating zone for 2.5 minutes, the heating zone being maintained at 500° C. by passing hot air upward through the belt, or at 400° C. for 4 minutes.
U.S. Pat. No. 5,444,034 relates to silver catalyst preparation wherein a support is impregnated with a hydrocarbon solution of a silver salt of an organic acid and activated in stages up to a temperature of 500° C. under an inert gas such as nitrogen.
In other descriptions of processes of ethylene oxide production, addition of oxygen-containing gases to the feed increased the efficiency. In U.S. Pat. No. 5,112,795, for example, 5 ppm of nitric oxide was added to the gas feed of composition: 8 volume % oxygen, 30 volume % ethylene, about 5 ppm ethyl chloride and the balance nitrogen.
U.S. Pat. Nos. 5,504,052 and 5,646,087 show silver catalysts for ethylene oxidation to ethylene oxide prepared by impregnating an inert support with a silver/amine solution as well as with various promoters and calcining the impregnated support at a range between 300° C. to 500° C., while the catalyst is maintained under an inert atmosphere.
In the other processes to increase efficiency, particularly selectivity, the silver catalysts were treated at a certain temperature and a certain gas mixture. For example in U.S. Patent Application Publication No. 2004/0049061 and U.S. Pat. No. 7,102,022 the selectivity of a highly selective epoxidation catalyst can be improved by heat-treating the catalyst in the presence of oxygen at a temperature which is typically above the catalyst's normal initial operation temperature. Typically, the heat-treating temperature is greater than 250° C. and contact occurs for a period of time up to 150 hours. Such treatment, which is performed under conditions different than regular production, will necessarily interfere with productivity of the given operation leading to reduced profitability of the plant.
U.S. Patent Application Publication No. 2007/0185339 discloses a process for treating a supported epoxidation catalyst which comprises contacting the catalyst or a precursor of the catalyst comprising silver in cationic form with a treatment feed comprising oxygen at a catalyst temperature of at least 350° C. for a duration of at least 5 minutes.
Thus, the prior art teaches catalyst preparation by calcining an impregnated support either in air, i.e., a large amount of oxygen, or under an inert atmosphere such as nitrogen.
In U.S. Patent Publication No. 2006/0252639, it is disclosed that the calcination of an impregnated support in an inert atmosphere, such as nitrogen, with the addition of only a small amount of an oxidizing gas, such as molecular oxygen in the inert atmosphere, improves the effective life, activity and selectivity of an ethylene oxide catalyst.
Specifically, the '639 publication provides a procedure for the preparation of a catalyst useful for the vapor phase production of ethylene oxide from ethylene and oxygen which comprises impregnating an inert support with a solution comprising a catalytically effective amount of a silver containing compound, a promoting amount of an alkali metal containing compound, and a promoting amount of a transition metal containing compound; calcining the impregnated support by heating the impregnated support at a temperature of from about 200° C. to about 600° C. for a time sufficient to convert the silver in the silver containing compound to metallic silver and to decompose and remove substantially all organic materials; the heating being conducted under an atmosphere comprising a combination of an inert gas and from about 10 ppm to about 5% by volume of a gas of an oxygen containing oxidizing component.
Although the calcination described in the '639 publication provides improvements over prior art calcination processes, indiscriminate addition of oxygen to an inert gas might, under some circumstances, lead to undesirable combustion rather than thermal decomposition of the catalyst precursor with negative consequences for catalyst performance. Moreover, in order to prevent open flame combustion in the system during calcination, the calcination process disclosed in the '639 publication is limited to 5 vol. % of an oxygen containing oxidizing component. This, depending on the specific formulation of the silver catalysts, may not be sufficient to fully and efficiently condition the catalyst.
In view of the above, there is a need for providing a calcination procedure that improves catalyst performance without experiencing any problems with respect to open flame combustion. Also, there is a need for providing a calcination procedure that requires no special treatment or time to start-up the catalyst which would result in improved economics for the operator.